Which of these numbers is composite? ${13,\ 29,\ 35,\ 71,\ 79}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 13, 29, 71, and 79 each have only two factors. The factors of 35 are 1, 5, 7, and 35. Thus, 35 is the composite number.